1. Field of the Invention
This invention relates to a spark plug insulator of an internal combustion engine and a method of making the same for use in an automobile and aircraft, and particularly a spark plug insulator which is improved to be superior in insulation and thermal-shock resistance.
2. Description of Prior Art
In an internal combustion engine, a spark plug insulator is exposed to the ambient temperature as high as 2000.degree. C. at an explosion stroke, and then exposed to an air-fuel mixture which has a temperature equivalent to the atmosphere at an intake stroke. This causes the insulator to be alternately subjected to a heat-and-cool cycle repeatedly so as to give the insulator repetitive thermal stress. This type of insulator has been made from a sintered ceramic material with aluminum oxide (alumina) as a main component.
With the recent demand of a high output with a high fuel efficiency of the internal combustion engine, it has been increasingly difficult to cope with an enhanced temperature of the combustion gas which causes a thermal shock on the insulator made of the aluminum oxide based ceramic material. It is found that the thermal shock finally induces cracks on the insulator made of the aluminum oxide based ceramic material depending on bench test conditions.
Therefore, it is an object of the invention to provide a spark plug insulator which is capable of improving a thermal-shock resistance due to repetitive thermal stress so as to prevent cracks on the insulator.